The present invention relates to a process for preparing diisocyanates from diamines and phosgene in the gas phase.
The phosgenation of amines to isocyanates in the gas phase is known, for example—in addition to numerous other documents—from EP 570799, EP 749 958 B1 and EP 1078918 B1.
In the known gas phase phosgenation processes, phosgene is evaporated before use in the process. This entails storage of liquid phosgene as a reservoir for use in the process and the supply of energy for evaporation.
Owing to the high toxicity of the phosgene, the aim for safety reasons is to keep the reservoir of phosgene and the amount of phosgene which are each present in the process stages (holdup) as low as possible. This aim is countered by the superstoichiometric use of the phosgene in the conversion of amine, such that considerable amounts of phosgene have to be recycled owing to the reaction.
It was an object of the present invention to provide a process for preparing diisocyanates, with which the amount of phosgene in the individual process stages of the diisocyanate preparation can be reduced.